JP2005179483A - Regeneration method of catalyst for thf polymerization - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To regenerate industrially advantageously with a mild condition a solid acid catalyst used for polymerization of THF. <P>SOLUTION: The solid acid catalyst used for polymerization of THF is treated with heat in the presence of a ≥2C carboxylic acid. Preferably, in the regeneration method of THF polymerization catalyst, the solid acid catalyst is a complex oxide of metals. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

The present invention relates to a method for regenerating a solid acid catalyst which is used for the polymerization of cyclic ether containing tetrahydrofuran (hereinafter abbreviated as THF) and which is at least partially deactivated.

  Polyether polyol is an industrially useful polymer as a raw material for soft segments such as elastic fibers and thermoplastic elastomers. Among them, polytetramethylene ether glycol (PTMG) synthesized by polymerization of tetrahydrofuran is attracting attention because of its excellent stretchability and elasticity. In order to further improve the low temperature elasticity and the like of PTMG, various copolymer products such as a copolymer product of 3-methyltetrahydrofuran and THF and a copolymer product of neopentyl glycol and THF have been developed. Polymerization or copolymerization catalysts for THF have also been studied earnestly, especially as solid acid catalysts that are very easy to separate and recover from products, such as zeolites, clay compounds such as activated clay, metal composite oxides, Examples include strongly acidic ion exchange resins. However, since the solid acid catalyst is gradually deactivated as the use time increases, or the product is colored or the like, it must be regenerated after being used for a certain period. The cause of the deactivation and coloring is presumed to be a polymerization reaction product that gradually adheres on the catalyst. As a method for regenerating the catalyst, a method of firing at a high temperature is conventionally known. For example, when the montmorillonite catalyst used in the THF polymerization is calcined at 650 to 850 ° C., it is reported that the attached organic matter can be removed, the activity is not only restored, but also an activity superior to the initial activity can be exhibited ( Patent Document 1). However, when firing at a high temperature on an industrial scale, firing is often performed with air that is readily available, and when the catalyst used for THF polymerization or copolymerization is fired in air, the attached organic matter is burned. Since it detaches, it becomes a very dangerous situation. In addition, an explosive mixture may be formed by air and desorbed organic vapor, which is unsuitable for industrial implementation.

Further, steam treatment has been reported as a method for removing organic substances adhering to the catalyst by THF polymerization at a lower temperature (see Patent Document 2). Here, a method is disclosed in which treatment is carried out with water vapor of 0.5 to 40 bar at 80 ° C. to 250 ° C., followed by baking at 400 to 800 ° C. However, when the treatment is performed under the above conditions, for example, in the case of zeolite containing aluminum, aluminum is desorbed, so that the composition of the catalyst itself changes. Although this treatment is relatively low temperature, it is very close and severe to hydrothermal treatment, so it is often not applicable depending on the catalyst.
JP-A-5-245385 Special Table 2002-535420 Publication

  An object of the present invention is to provide a method for regenerating a THF polymerization catalyst under mild conditions that can be easily carried out on an industrial scale.

As a result of intensive studies in view of the above circumstances, the present inventors have determined that the THF polymerization reaction is an equilibrium reaction between a polymerization reaction and a depolymerization reaction, and only the depolymerization reaction proceeds at a ceiling temperature (85 ° C.) or higher. Therefore, it has been found that the organic matter can be removed under relatively mild conditions as compared with the prior art by heating to near the ceiling temperature or higher in a carboxylic acid solvent that dissolves the attached organic matter well.
That is, the gist of the present invention resides in a method for regenerating a THF polymerization catalyst, wherein the solid acid catalyst used for the polymerization of THF is subjected to a heat treatment in the presence of a carboxylic acid having 2 or more carbon atoms.

  According to the method of the present invention, the THF polymerization catalyst can be regenerated industrially advantageously under mild conditions without requiring high-temperature calcination.

<Solid acid catalyst>
The solid acid catalyst which is the subject of the present invention is used for the polymerization of THF, and the polymerization of THF here only needs to polymerize a monomer containing THF as a raw material, and only homopolymerization of THF. The solid acid catalyst that includes copolymerization of THF and other monomers is not particularly limited as long as it is known as a catalyst for the production of polyether polyols. Is mentioned.

a) Group 3-14 elements of the periodic table, preferably {Ge, Sn, Pb, B, Al, Ga, Zn, Cd, Cu, Fe, Mn, Ni, Cr, Mo, W, Ti, Zr, Hf , Y, La, Ce, Yb and Si}, a catalyst obtained by subjecting one type and / or two or more types of metal oxides selected from the group consisting of Si} to a high temperature of about 600 ° C. to 1100 ° C.
b) A catalyst obtained by adding a treatment such as acid treatment to a clay compound containing Si and Al, for example, a clay compound such as montmorillonite, saponite, sepiolite, mica and a viscosity compound such as activated clay.

c) Zeolite having a structure selected from the group consisting of BEA, EMT, ERI, EUO, FAU, HEU, LTA, LTL, MAZ, MOR, MTW, NES, OFF, TON (Structure Commission of the International)
Zeolite Association's “ATLAS OF ZEOLITE STRUCTURE TYPES, Third Edition (1992), W. Meier and DH Olson” and strongly acidic ion exchange resins.

These may be used alone or in combination of two or more.
Examples of the catalyst that can be used repeatedly and stably include the metal oxide a) and the clay compound b) or a catalyst obtained by the treatment thereof. Among these, a metal composite oxide containing two or more elements in {} described above is preferable. Specifically, Zr—Si, Sn—Si, Al—Si, Fe—Si, Ge—Si, Mo—Si, W-Si and Ti-Si are mentioned.
Among them, a Zr—Si composite oxide is particularly preferably used.

<Method of polymerizing THF>
The applicable range of the present invention is homopolymerization containing THF and copolymerization with other monomers. As other monomers, alcohols and cyclic ethers are preferably used.
The alcohols preferably have two or more hydroxyl groups, such as ethylene glycol, 1,2-propylene glycol, 1,3-propylene glycol, 1,6-hexanediol, 2-methyl-1,3. -C2-C8 diols such as propanediol and neopentyl glycol, C3-C8 triols such as glycerin and trimethylolpropane, C4-C8 tetraols such as erythritol, etc. .

  Examples of the cyclic ethers include oxiranes such as ethylene oxide and 1,2-propylene oxide, oxetanes such as oxetane, 3-methyloxetane, and 3,3-dimethyloxetane, 3-methyltetrahydrofuran, and 3,4-dimethyltetrahydrofuran. Examples thereof include oxolanes, dioxolanes such as 1,3-dioxolane and 2-methyl-1,3-dioxolane, oxepanes such as oxepane and 3-methyloxepane, and lactones such as γ-butyrolactone and ε-caprolactone.

Moreover, the usage-amount of a copolymerization monomer component is 50 mol% or less normally in all the monomers, Preferably it is 20 mol% or less.
Preferred copolymerization components include cyclic ethers due to their high polymerization reactivity, especially oxiranes, oxetanes, and oxolanes. The inclusion of these improves elastic properties, particularly stretchability at low temperatures. A polyether polyol suitable for the formed elastic fiber and thermoplastic resin raw material is obtained.

During the polymerization reaction of THF, water, alcohols, carboxylic acids and / or anhydrides thereof can be used as a reaction initiator and a termination agent. As the alcohol, the same kind as the above-mentioned copolymerization monomer is used. Examples of the carboxylic acid include aliphatic or aromatic carboxylic acids having a lower limit of usually 2 or more and an upper limit of usually 12 or less, preferably 8 or less. The carboxylic acid is preferably a monocarboxylic acid, but polycarboxylic acids can also be used. Considering the effect, price, and availability, it is preferable to use acetic acid or acetic anhydride. The amount of these initiators and terminators used can be selected over a wide range depending on the physical properties such as the molecular weight of the desired polyether polyol, the desired reaction rate, etc., but the lower limit is usually 0.1% relative to the total amount of raw material monomers. It is suitable that it is at least mol%, preferably at least 1 mol%, and the upper limit is usually at most 50 mol%, preferably at most 30 mol%.
The polymerization or copolymerization reaction can be carried out either batchwise or continuously.

  If the solid acid catalyst is a batch system, the lower limit is usually 0.001 times by weight or more with respect to the liquid phase, and the upper limit is usually 50 times by weight or less, preferably 20 times by weight or less. If the amount of the catalyst is too small, the polymerization reaction is slowed. On the other hand, if the amount is too large, it is difficult to remove the heat of polymerization, and the slurry concentration in the reaction system becomes high. Also prone to problems. The continuous reaction can be carried out by either a suspension bed system or a fixed bed flow system. In the case of fixed bed flow, the catalyst has a lower limit of usually 0.001 times by weight or more and an upper limit of usually 1000 times by weight or less, preferably 500 times by weight or less with respect to the liquid phase supply amount per unit time. Chosen from.

  The reaction is usually carried out without solvent, but a solvent can be used if desired. Any solvent may be used as long as it does not inhibit the reaction. For example, hydrocarbons such as hexane, heptane, and octane, halogenated hydrocarbons such as dichloromethane and chloroform, and linear ethers such as diethyl ether and dibutyl ether. , Aromatic hydrocarbons such as benzene, toluene, xylene and the like, and one kind or two or more kinds may be used in combination.

  The lower limit of the reaction temperature is usually −20 ° C. or higher, preferably 0 ° C. or higher, and the upper limit is usually 100 ° C. or lower, preferably 60 ° C. or lower. The reaction pressure may be any pressure that allows the reaction system to maintain a liquid phase, and is usually selected from the range of normal pressure to 10 MPa, preferably 5 MPa. The reaction time is not particularly limited, but the lower limit is usually 0.1 hours or longer, preferably 0.5 hours or longer, and the upper limit is usually 20 in consideration of the amount of catalyst, the yield of the polymer, and the economy. It is selected from the range of not more than time, preferably not more than 15 hours. The reaction time here refers to the time from the time when the predetermined reaction temperature is reached until the reaction is completed until the catalyst is deactivated in the batch method, and in the continuous method, the reaction composition in the reactor. It refers to the residence time of the liquid.

The reaction atmosphere is under an inert gas, and for example, nitrogen, argon, or helium is used.
<Method for separating catalyst>
After completion of the reaction, the catalyst is separated by a general separation method for a solid catalyst. Examples thereof include filtration and decantation. Specifically, in the case of batch reaction, after completion of the reaction, an extraction tube having a filter at the tip is placed in the reactor, the reaction liquid is extracted while filtering the catalyst, and the catalyst is left in the reactor. Thus, the reproduction process can be performed as it is. Moreover, it can stand still after completion | finish of reaction, the process in a reactor is possible like the above by depositing a catalyst and collect | recovering supernatants.

<Catalyst regeneration method>
The catalyst separated and recovered after the reaction is usually attached with an organic substance such as a polymerization product, and if used as it is, the activity is low, and the product may be colored, so it must be regenerated.
In the present invention, the attached catalyst is removed by subjecting the used catalyst to heat treatment in the presence of carboxylic acid.

  Examples of the carboxylic acid to be used include aliphatic or aromatic carboxylic acids having a lower limit of 2 or more and an upper limit of usually 12 or less, preferably 8 or less. The carboxylic acid is preferably a monocarboxylic acid, but polycarboxylic acids can also be used. Specifically, as the aliphatic carboxylic acid, acetic acid, propionic acid, butyric acid, valeric acid, caproic acid, heptanoic acid, caprylic acid, pelargonic acid, maleic acid, succinic acid, etc. are used, and these mixed carboxylic acids Can be used. As the aromatic carboxylic acid, benzoic acid, phthalic acid, naphthalic acid or the like is used. In the present invention, the boiling point of the carboxylic acid is preferably near the ceiling temperature of the THF polymerization reaction, and if the boiling point is too high, it is difficult to remove from the catalyst. Therefore, among the above, aliphatic monocarboxylic acids are preferable, and among these, it is particularly preferable to use acetic acid in consideration of the effect, cost, and availability.

  The amount of carboxylic acid used depends on the amount of attached organic matter, that is, the usage record of the catalyst, but is preferably at least the amount that the catalyst is completely immersed. Specifically, the lower limit is usually 0.5 times or more, preferably 1 or more times, more preferably 2 or more times the catalyst amount, and the upper limit is usually 100 times or less, preferably 50 times. It is not more than twice, more preferably not more than 10 times by weight.

The heat treatment temperature with carboxylic acid is preferably near the ceiling temperature of the THF polymerization reaction because depolymerization tends to proceed, and the lower limit is usually 80 ° C. or higher, preferably 85 ° C. or higher, more preferably 90 ° C. The upper limit is usually 200 ° C. or lower, preferably 180 ° C. or lower, and more preferably 150 ° C. or lower.
Although the treatment time depends on the actual use of the catalyst, the lower limit is usually 0.2 hours or more, preferably 0.5 hours or more, more preferably 1 hour or more, and the upper limit is usually 10 hours or less, preferably 8 hours or less. More preferably, it is 5 hours or less. In addition, when the deactivation of the catalyst is large (the amount of attached organic matter is large), it is also effective to repeat the above heat treatment twice or more. Specifically, after carrying out the heat treatment under the above conditions, the catalyst and the carboxylic acid are separated by a general method such as filtration and decantation, and a necessary amount of new carboxylic acid is added and heated under the above conditions. Processing can be performed. In many cases, 2 to 5 repetitions are sufficient.

After completion of the heat treatment, the catalyst is obtained by separating the carboxylic acid by filtration, decantation or the like. Thereafter, it can be used as it is for the polymerization or copolymerization reaction of THF, or it can be used after being washed at least once with one kind of reaction substrate such as THF or a mixed solution. It is also possible to wash with other solvents. Further, it can be dried in an inert gas such as nitrogen, helium or argon at room temperature or dried or dried under reduced pressure, and then used again for the polymerization reaction of THF. Among them, it is preferable to wash with alcohol in order to suppress coloring of the obtained THF polymer and to use it stably and repeatedly.

<Alcohol cleaning method>
The alcohol used here is difficult to remove from the catalyst if the boiling point is too high. Specifically, the lower limit of the carbon number is usually 1 or more, and the upper limit is usually 10 or less, preferably 8 The following aliphatic alcohols are mentioned. The alcohol is preferably a monoalcohol, but polyols can also be used. Specifically, methanol, ethanol, propanol, butanol or the like is used as the alcohol, and a mixed alcohol can be used. Among these alcohols, it is preferable to use methanol in consideration of effects, price, and availability.

  The amount of methanol used is only required to be able to remove the carboxylic acid still adhering after the separation, and is preferably equal to or more than the amount in which the catalyst is completely immersed. Specifically, the lower limit is usually 0.5 times or more, preferably 1 or more times, more preferably 2 or more times, and the upper limit is usually 100 or less, preferably 50 or less times the catalyst amount. More preferably, it is 10 weight times or less.

The lower limit of the temperature during alcohol washing is usually 0 ° C. or higher, and the upper limit is usually 200 ° C. or lower, preferably 150 ° C. or lower.
The lower limit of the treatment time is usually 0.2 hours or longer, preferably 0.5 hours or longer, and the upper limit is usually 5 hours or shorter, preferably 4 hours or shorter, preferably 3 hours or shorter. It is also effective to repeat the above process twice or more. Specifically, after the treatment is performed under the above conditions, the catalyst and the alcohol are separated by a general method such as filtration and decantation, and the necessary amount of new alcohol is added and the heat treatment is performed under the above conditions. can do. In many cases, 2 to 5 repetitions are sufficient.

  After the treatment, the catalyst is obtained by separating the alcohol by filtration, decantation or the like. Thereafter, it can be used as it is for the polymerization or copolymerization reaction of THF, or it can be used after being washed at least once with one kind of reaction substrate such as THF or a mixed solution. It is also possible to wash with other solvents. Furthermore, it can be used for the polymerization or copolymerization reaction of THF after drying at room temperature or heating in an inert gas such as nitrogen, helium and argon, or drying under reduced pressure. Among them, it is preferable to dry after this for relatively high activity and stable repeated use.

<Drying method>
As a drying method, it may be dried while circulating an inert gas such as nitrogen, helium, or argon, or may be dried under reduced pressure. Although the drying conditions depend on the boiling point of the alcohol used, etc., for example, when drying while circulating an inert gas, it is preferable to heat above the boiling point of the alcohol.

When the drying temperature is heated to a high temperature, the load on the apparatus increases. Therefore, the lower limit is usually 0 ° C. or higher, and the upper limit is usually 200 ° C. or lower, preferably 180 ° C. or lower.
The lower limit of the treatment time is usually 0.2 hours or longer, preferably 0.5 hours or longer, and the upper limit is usually 10 hours or shorter, preferably 8 hours or shorter. In the case of drying under reduced pressure, it is usually carried out at 600 mmHg or less, preferably 300 mmHg or less, more preferably 100 mmHg or less, and the above temperature and time.

  It is preferable that the amount of organic substances finally adhered to the catalyst is usually 15% by weight or less, more preferably 10% by weight or less after the above treatment.

EXAMPLES Specific examples of the present invention will be described below in more detail with reference to examples. However, the present invention is not limited to the following examples unless it exceeds the gist.
The apparatus and conditions for gel permeation / chromatography (GPC) analysis and NMR analysis used in this example are as follows.
<GPC analysis>
Column: TSK-GEL GMH _XL- N (7.8 mm ID × 30.0 cm L) (manufactured by Tosoh Corporation)
Mass calibration: Polytetrahydrofuran calibration kit (manufactured by Polymer Laboratories)
(M _p = 547000, 283000, 99900, 67500, 35500,
(15000, 6000, 2170, 1600, 1300)
Solvent: Tetrahydrofuran <APHA colorimeter analysis>
Device: CorrMeter ZE2000 (Nippon Denshoku Industries Co., Ltd.)
<NMR analysis>
Device: AVANCE400 (BRUKER)
Solvent: chloroform-d

Reference Example 9 Preparation of 9 mol% ZrO ₂ —SiO ₂ Composite Oxide Catalyst A 9 mol% ZrO ₂ —SiO ₂ composite oxide catalyst was prepared by the following method with reference to JP-A-9-94464. 10.0 g silica (Fuji Silysia product, Caractect Q-15, average pore size 150 mm, particle size 300 mesh or less), containing 4.34 g zirconium oxynitrate dihydrate and 2.11 g urea Add to 37 ml of aqueous solution. Water was distilled off under reduced pressure at 60 ° C. to obtain an apparently dried solid. Under a stream of air, the solid was heated to 120 ° C. over 1 hour at a constant heating rate, and subsequently heated to 800 ° C. over 2 hours 30 minutes. The mixture was kept at 800 ° C. for 3 hours and then allowed to cool.

Example 1
A glass container equipped with a stirrer and a thermometer was purged with nitrogen, and 66.8 g of THF, 0.20 g of 3-ethyl-3-methoxymethyloxetane and 6.7 g of acetic anhydride were added, and the water temperature was stirred while stirring in a nitrogen atmosphere. Was adjusted to 40 ° C. in a controlled water bath. While stirring the reaction solution, 4.1 g of ZrO ₂ —SiO ₂ prepared in Reference Example 1 was added, and 7.2 g of 3-ethyl-3-methoxymethyloxetane was added dropwise at a constant rate over 2 hours while the reaction solution temperature was changed. The reaction was carried out at 40 ° C. and normal pressure for 3 hours.

After completion of the reaction, about 1 ml of the reaction solution was filtered with a chromatographic disk, and a part was prepared as a 0.1 wt% THF solution as a GPC measurement sample, and a part was dissolved in chloroform-d to prepare an NMR sample. Thereafter, the mixture was allowed to stand for 20 minutes, and when the catalyst was precipitated, the supernatant was collected and filtered, and then THF was collected under reduced pressure and measured with a colorimeter. An operation of adding 66.8 g of THF to the catalyst, stirring for 10 minutes at room temperature, and extracting THF using a ball filter was performed twice. Thereafter, 66.8 g of acetic acid was added, and after gently refluxing for 2 hours, acetic acid was extracted with a ball filter. Thereafter, 66.8 g of methanol was added, stirred for 10 minutes at room temperature, and extracted with a ball filter twice. Then, the reactor was immersed in an oil bath heated to 150 ° C. while flowing nitrogen through the reactor for 2 hours. Dried. At this point, the organic matter adhering to the catalyst was 2% by weight of the catalyst.

To the obtained catalyst, THF 66.8 g, 3-ethyl-3-methoxymethyloxetane 0
. A mixture of 20 g and 6.7 g of acetic anhydride warmed to 40 ° C. was added, and the mixture was placed in a water bath adjusted to 40 ° C. while stirring under a nitrogen atmosphere. While 7.2 g of 3-ethyl-3-methoxymethyloxetane was added dropwise at a constant rate over 2 hours, the reaction was carried out for the second time at 40 ° C. under normal pressure for 3 hours, and the above operation was repeated for 9 times in total. Was repeated. Table 1 shows the conversion rate, GPC analysis results, and colorimeter analysis results. The yield was calculated by the following formula using the NMR results.

THF conversion (%) = (integral value of 1.62 ppm peak) / (integrated value of 1.62 ppm peak + integral value of 1.85 ppm peak) × 100
3-ethyl-3-methoxymethyloxetane (%) = {0.84 ppm peak integrated value− (4.40 ppm peak integrated value + 4.48 ppm peak integrated value +) / 4 × 3} / (0 .84ppm peak integration value) x 100

Peak assignment 1.62 ppm: Methylene group not adjacent to oxygen in tetrahydrofuran unit in polymer 1.85 ppm: Methylene group not adjacent to oxygen in tetrahydrofuran 0.84 ppm: 3-ethyl-3-methoxymethyloxetane unit in polymer and 3-
Methyl group of ethyl-3-methoxymethyloxetane 4.40 ppm, 4.48 ppm: methylene group adjacent to oxygen on 4-membered ring of 3-ethyl-3-methoxymethyloxetane

From this, it is clear that the reaction after the second time is very stable and can be repeatedly used with almost the same result without performing high-temperature baking.
Example 2
After ZrO ₂ —SiO ₂ prepared in Reference Example 1 was used in a continuous polymerization reactor of THF and acetic anhydride, 4.1 g of the catalyst which was extracted, washed with THF and dried at 40 ° C. in a nitrogen stream was stirred with a thermometer. And 66.8 g of acetic acid was added, and heat treatment, methanol washing and drying were carried out in the same manner as in Example 1. Thereafter, tetrahydrofuran heated to 40 ° C. 66
. A mixture of 8 g and 6.7 g of acetic anhydride was added, and the mixture was placed in a water bath with the water temperature adjusted while stirring under a nitrogen atmosphere to 40 ° C. The reaction was carried out at normal pressure for 3 hours. After completion of the reaction, sampling was performed in the same manner as in Example 1. The results are shown in Table-2.

Comparative Example 1 (Example in which heat treatment and methanol washing in the presence of acetic acid were not performed in Example 2)
After ZrO ₂ —SiO ₂ prepared in Reference Example 1 was used in a continuous polymerization reactor of THF and acetic anhydride, 4.1 g of the catalyst which was extracted, washed with THF and dried at 40 ° C. in a nitrogen stream was stirred with a thermometer. In the same manner as in Example 1, it was immersed in an oil bath at 150 ° C. in a nitrogen stream and dried for 2 hours. Thereafter, 66.8 g of THF warmed to 40 ° C., 6.7 g of acetic anhydride
The mixture was put into a water bath with adjusted water temperature while stirring under a nitrogen atmosphere, and the temperature was adjusted to 40 ° C. The reaction was carried out at normal pressure for 3 hours. After completion of the reaction, sampling was performed in the same manner as in Example 1. The results are shown in Table-2.

  From the above, it is clear that the activity of the deactivated catalyst is greatly improved and the coloring is greatly reduced by the cleaning method of the present invention.

  According to the method of the present invention, the solid acid catalyst used for the polymerization of THF can be regenerated industrially advantageously under mild conditions without requiring ordinary high-temperature calcination.

Claims (5)

A method for regenerating a THF polymerization catalyst, comprising subjecting a solid acid catalyst used for polymerization of tetrahydrofuran (hereinafter abbreviated as THF) to heat treatment in the presence of a carboxylic acid having 2 or more carbon atoms.   The method for regenerating a THF polymerization catalyst according to claim 1, wherein the solid acid catalyst is a complex oxide of metal.   The method for regenerating a THF polymerization catalyst according to claim 1 or 2, wherein the heat treatment temperature is 80 ° C or higher.   The method for regenerating a THF polymerization catalyst according to any one of claims 1 to 3, wherein the solid acid catalyst is separated from the carboxylic acid after the heat treatment, and then the solid acid catalyst is contacted with an alcohol having 1 or more carbon atoms. The method for regenerating a THF polymerization catalyst according to claim 4, wherein after the contact treatment with the alcohol, the solid acid catalyst is separated from the alcohol, and then the solid acid catalyst is dried.